1. Field of the Invention
The present invention relates to the design, configuration and construction of improved ventilated hydrofoils and their use in wind and motor driven watercraft.
2. Description of the Related Prior Art
Hydrofoils are widely used in both motor and wind powered water craft with the aim of reducing drag and/or improving passenger comfort by lifting the hull of the craft out of the water. However, hydrofoil craft face difficulties in locating the craft at a specified distance above the water surface. They can also suffer from inconsistent behaviour due to cavitation and ventilation and, if this is reduced by careful section design, can have a very narrow operating speed range unless moving parts such as flaps are introduced. There are number of solutions to these problems but each introduces difficulties. The present invention solves these issues in an innovative manner and avoids many of the related problems.
The simplest ride height control method is to use either a ladder of hydrofoil lifting surfaces or an inclined hydrofoil that pierces the surface. In each case as the speed of the craft increases more lift is generated and the craft rises. As this happens either one “rung” of the ladder of hydrofoils is lifted clear of the water reducing lift, or a portion of the inclined foil rises out of the water. In either case reduced lifting area produces a reduction in total lift. This carries on as the craft rises until equilibrium is reached and the craft rises no further. Though simple and robust this solution has a number of undesirable characteristics. In the case of the inclined hydrofoils, sections that are optimal when fully immersed, are sub-optimal at the water surface and produce undesirable characteristics as they pass through it—such as unwanted ventilation and spray drag. For ladder foils as well as the above difficulties the multiple small hydrofoils and junctions produce additional drag at low speed in a fully immersed condition.
Another approach to height control is to use fully immersed hydrofoils but to control the ride height by varying their lift under the control of a surface sensor—either mechanical or electrical. This involves additional complexity as well as long vertical legs attaching the hull to the lifting hydrofoils. It is difficult for mechanical systems to control height accurately in the presence of large waves and varying loads. Additionally sections with high lift to drag ratios cavitate at high speeds so reducing their lift.
Occasionally super ventilating surface running hydrofoils have been used. These control the ride height directly as they run on the surface. However, they have high drag and low lift at low speed, they also may have undesirable pitching moment characteristics. The transition from unventilated to ventilated operation is often also associated with highly non-linear lift behaviour.
Since ventilated foils often need to have sharp, or otherwise very thin leading edge sections they are also vulnerable to damage and erosion. This mitigates against the use of simple fibre-composite construction.